It is common in modern jet powered aircraft to find separate systems which perform the thermal anti-icing (TAI) and environmental control functions, utilizing hot, compressed gas from the engine compressors called "bleed air." In such aircraft a portion of the total bleed air available is routed to the various components in the aircraft which are subject to icing and then exhausted overboard. The remainder of the bleed air is routed to the environmental control system (ECS) where it is used to heat (or cool) and pressurize the aircraft cabin. These systems may be referred to as "open" systems because each portion of bleed air used to de-ice a particular component, such as the wing leading edges or the engine inlet, is simply exhausted overboard even though all useful heat has not been extracted.
FIG. 1 is a schematic diagram of a typical open system combining the thermal anti-icing and environmental control functions. Engine bleed air is drawn from a suitable stage (or stages) of the engine compressors and is routed through a heat exchanger, called a pre-cooler, which is normally located somewhere in the engine nacelle. Cooling is generally accomplished by passing cool air from the engine fan stage source 14, through the pre-cooler 12, and exhausting it overboard. Flow of the cooling air is controlled by shut-off valve 16.
The bleed air, cooled to about 450.degree.F or lower by the pre-cooler, is now available for use in the various TAI sub-systems and in the cabin environmental control sub-system (ECS). Air flow into the inlet and acoustic ring (if any) TAI systems 18 is controlled by a shut-off and pressure regulator valve 20. Further downstream, the bleed air must pass through a pressure regulator 22 before flowing into the wing TAI system 24 or the environmental control system 28. Flow into the wing TAI system 24 is controlled by shut-off valve 26. Flow entering the environmental control system 28 passes first through shut-off valve 30, then through flow control valve 32, through an air cycle machine 34, and into the aircraft cabin 36 before being exhausted overboard. While the aircraft is on the ground, cabin air may be supplied through duct 38 from an auxiliary power unit (APU).
It has been found that the extraction of bleed air from high-bypass-ratio turbofan engines imposes a significant penalty on engine performance, so it is desirable to minimize bleed air requirements as much as possible. Modern wide-bodied jet transports impose large bleed air demands on their engines, and the addition of noise-suppressing devices, such as acoustic rings, in the engine air inlets which require anti-icing may impose unacceptable demands on the engine. One possible solution to this problem is to make more efficient use of bleed air heat with an improved, integrated thermal anti-icing and environmental control system as described herein.
Most present systems route a portion of the total available bleed air to each of the TAI sub-systems and then discard each portion overboard, even though it may still contain usable heat. It may be impractical to attempt to recover heat from certain anti-icing systems because of their locations or because of the additional weight which would be added by the recovery ducts. On the other hand, where certain systems are located in relatively close proximity to each other, it is possible to pass bleed air sequentially from one system to the next until most of the useful heat is extracted. Present systems also waste bleed air heat by passing the bleed air through an initial pre-cooling unit such as pre-cooler 12 shown in FIG. 1. According to this invention, the pre-cooler is eliminated and its function is performed by the inlet TAI system which uses the heat ordinarily wasted by the pre-cooler.
Another means for conserving bleed air and bleed air energy found in one particular embodiment of this invention is a closed-loop liquid anti-icing system for acoustic rings. Most rings are inherently difficult to heat internally with air because of their geometry. The air passages must be narrow and complex, and as a result, high pressure losses occur in the bleed air. In this embodiment, these losses are avoided by the use of closed-loop liquid anti-icing system wherein heat is transferred to the liquid in a heat exchanger from bleed air which is previously passed through the inlet TAI system.
The use of bleed air as a source of heat for de-icing is well known in the art. A typical such use is described in U.S. Pat. No. 3,341,114 to H.A. Larson dated Sept. 12, 1967, wherein bleed air is ducted through the inlet guide vanes of a gas turbine engine for de-icing. It is also common to use engine bleed air from multiplicity of tasks on the same aircraft. In U.S. Pat. No. 2,777,301 to J. Kuhn dated Jan. 15, 1957, a power and air-conditioning system is disclosed which uses engine bleed air and includes an air cycle machine. In this patent, it is suggested that bleed air may be ducted away from the system for use in de-icing systems and other accessories.
It is also common to de-ice the surfaces of various aircraft components by circulating a hot liquid through them. In spite of the better heat transfer characteristics obtainable with liquids such systems have seen limited use in aircraft as compared to air circulating systems because of increased weight and leakage problems. A system for de-icing intake components which may use either engine lubricant or coolant is described in British Pat. No. 629,044. In this system liquid is circulated by a pump through the various parts to be de-iced and these parts act simultaneously as radiators to cool the liquid before it is returned to the engine.